BtoB 4U (Korean: 비투비 포유; RR: Bitubi poyu; stylized in all caps) is the second official sub-unit of South Korean boy band BtoB formed under Cube Entertainment.

The sub-group consisted of members who have completed their military service Eunkwang, Minhyuk, Changsub and Peniel, who is exempt from conscription.

[1] The quartet’s first activity as an unofficial unit was the Naver Now show BTOB NOW: Our Concert, which was hosted by Eunkwang, Minhyuk, Changsub and Peniel, and was aired for 3 weeks starting October 27.

[2] Throughout the broadcast, the quartet performed numerous songs on 'BtoB Masterpiece One Bar Live' and communicated freely with fans through 'Real-time Q&A'.

[10][8] On October 30, Cube Entertainment officially announced the unit’s debut date with the album Inside.